Chondrocytes that were isolated from adult human articular cartilage changed phenotype during monolayer tissue culture, as characterized by a fibroblastic morphology and cellular proliferation. Increased proliferation was accompanied by downregulation of the cartilage‐specific extracellular matrix proteoglycan, aggrecan, by cessation of type‐II collagen expression, and by upregulation of type‐I collagen and versican. This phenomenon observed in monolayer was reversible after the transfer of cells to a suspension culture system. The transfer of chondrocytes to suspension culture in alginate beads resulted in the rapid upregulation of aggrecan and type‐II collagen and the downregulation of expression of versican and type‐I collagen. Type‐X collagen and osteopontin, markers of chondrocyte hypertrophy and commitment to endochondral ossification, were not expressed by adult articular chondrocytes cultured in alginate, even after 5 months. In contrast, type‐X collagen was expressed within 2 weeks in a population of cells derived from a fetal growth plate. The inability of adult articular chondrocytes to express markers of chondrocyte hypertrophy has underscored the fundamental distinction between the differentiation pathways that lead to articular cartilage or to bone. Adult articular chondrocytes expressed only hyaline articular cartilage markers without evidence of hypertrophy.

Thrombospondins (TSPs) are widely known as a family of five calcium-binding matricellular proteins. While these proteins belong to the same family, they are encoded by different genes, regulate different cellular functions and are localized to specific regions of the body. TSP-5 or Cartilage Oligomeric Matrix Protein (COMP) is the only TSP that has been associated with skeletal disorders in humans, including pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED). The pentameric structure of COMP, the evidence that it interacts with multiple cellular proteins, and the recent reports of COMP acting as a 'lattice' to present growth factors to cells, inspired this review of COMP and its interacting partners. In our review, we have compiled the interactions of COMP with other proteins in the cartilage extracellular matrix and summarized their importance in maintaining the structural integrity of cartilage as well as in regulating cellular functions.

Members of the interleukin-17 cytokine family are present in a variety of tissues (1-3), although the founding member, interleukin-17, is expressed exclusively in T cells and B cells (4-8). The cloning and characterization of a novel single-pass transmembrane protein with limited homology to the interleukin-17 receptor is reported. High mRNA levels were detected in prostate, cartilage, kidney, liver, heart, and muscle, whereas transcripts were barely detected in thymus and leukocytes. At least 11 RNA splice variants were found, transcribed from 19 exons on human chromosome 3p25.3–3p24.1. Differential exon usage was found in different tissues by quantitative reverse transcriptase-PCR. Predicted proteins range from 186 to 720 amino acids. Soluble secreted proteins lacking transmembrane and intracellular domains are predicted from several splice isoforms and may function as extracellular antagonists to cytokine signaling by functioning as soluble decoy receptors. Using antibodies directed at the cytoplasmic and extracellular domains of this protein, we investigated its localization and found that it was expressed in a variety of normal human tissues including prostate and in prostate cancer.